The invention relates to the field of the simultaneous reception/transmission of signals in a transmission system with a transmitter and a receiver. It relates more particularly to a device for receiving/transmitting signals in a transmission system linking a terminal comprising the device according to the invention with a satellite or a terrestrial base station.
Nowadays, remote transmission systems implementing the technique known as frequency division bidirectional transmission (xe2x80x9cFrequency Division Duplexxe2x80x9d or xe2x80x9cFDDxe2x80x9d) are commonly used, but not exclusively, in the field of bidirectional satellite transmissions.
Since transmission and reception are simultaneous and since the level of the transmission signal is high in such a reception/transmission device, the reception path is disturbed by a parasitic signal created by coupling by the transmission signal transmitted over the transmission path owing to the considerable power at the output of the transmitter amplifier. This parasitic signal might saturate the input stage of the reception path (often consisting of a transistor whose drain current and bias voltage are low) and hence might disturb its proper operation. This problem is accentuated in the case of transmission employing digital modulations with a non-constant envelope exhibiting considerable instantaneous peak amplitudes, for example of the code division multiplex access type (xe2x80x9cCDMAxe2x80x9d). FIG. 1 diagrammatically represents the input/output architecture of a reception/transmission device, coupled to an antenna 1. The signal received by the antenna is transmitted to an input stage represented by a low-noise amplifier 2 (xe2x80x9cLNAxe2x80x9d) whereas the output stage is represented by a solid-state power amplifier 3 (xe2x80x9cSSPAxe2x80x9d) delivering the signal to be transmitted to the antenna. A dashed arrow diagrammatically shows the parasitic coupling between the two paths, which is created by the transmission signal on the reception signal.
To remedy this problem of parasitic coupling, a common technique consists in carrying out very effective filtering of the transmission signal so as to eliminate the coupling of the latter in the reception path. Conventionally, a diplexer including very selective band-pass filters makes it possible to isolate respectively the transmission and reception frequency bands. This solution has the advantage of effectively separating the two frequency bands. However, additional in-line losses are brought about at the input of the low-noise amplifier, resulting in a degradation of the noise factor of the reception path.
The invention avoids the prior art problems and to optimize the performance of the input stage of a signal reception/transmission device such as cited hereinabove.
To this end, the invention is a device for the simultaneous reception/transmission of signals in a transmission system with a transmitter and a receiver, the device comprising a reception path for receiving a reception signal occupying a reception frequency band and a transmission path for transmitting a transmission signal occupying a transmission frequency band, the said reception and transmission bands being disjoint, the reception path comprising a low-noise amplification stage comprising means for amplifying the reception signal, the operating characteristic of these means being linear for an amplitude range of the input signal and exhibiting saturation onwards of an amplitude of a given input signal, the said device being characterized in that
an optimal load impedance is defined for linear behaviour of the amplification means,
means are arranged at the output of these amplification means for referring back to this output on the one hand an open circuit in the reception frequency band and on the other hand an impedance equal to the optimal load impedance in the transmission frequency band.
Thus, the invention makes it possible to minimize the degradation brought about by the parasitic signal originating from the transmission path at the transmission frequency, which may be of relatively considerable power. The constraints relating to the filtering of the parasitic signals in the transmission band may then be less strict upstream of the reception chain. If the isolation between the transmission and reception paths is sufficient, there is no longer any need to carry out filtering at the input of the low noise amplification stage. In any event, the ohmic losses of the input filter are reduced due to the relaxing of the constraints on this filter, thus making it possible to improve the overall noise factor of the reception chain.
According to one embodiment, first filtering means arranged on the reception path are adapted for referring back to the output of the amplification means an open circuit in the transmission frequency band and an impedance matched to that of the transmission path in the reception frequency band.
According to one embodiment, the said first filtering means comprise a band-pass filter centred on the reception frequency and whose bandwidth is the reception frequency band.
According to one embodiment, the first filtering means comprise a band-stop filter which cuts off the transmission frequency band.
According to one embodiment, second filtering means, such as a band-stop filter, are arranged at the input of the amplification means for cutting off the transmission frequency band.
The invention concerns also a terminal for communication with at least one satellite or a terrestrial base station, of the type comprising an antenna, characterized in that it comprises the device according to the invention.